The present invention concerns passive reflectors for electromagnetic waves and more particularly passive reflectors of the Luneberg type. Such reflectors are well known to the person skilled in the art. They consist essentially of a dielectric sphere of which the index varies along a radius in accordance with a known law and which has on part of its surface a metallic coating serving to reflect the incident energy. Such a reflector has been described notably in U.S. Pat. No. 3,204,244 issued on June 25, 1963 and assigned to the same Assignor as the present application. These reflectors operate on rectilinearly polarized plane waves.
Such reflectors are often used to increase the equivalent surface of the targets employed for the purpose of monitoring the performances of radar systems, because they have a considerable equivalent surface with a small mass and an angular aperture which can be very considerable. Present-generation radar systems exit circularly polarized waves, inter alia for reducing the effects of raindrops. It is therefore necessary to provide targets which are capable of reflecting circularly polarized waves. It is known that a metallic surface reflects a circularly polarized wave in the form of a circularly polarized wave having a reverse direction of rotation. Taking into account the conventions employed to define the direction of rotation of circularly polarized waves (trirectangular trihedron of which the axis Ox is directed in the sense of the propagation), the electric fields of the transmitted and reflected waves are in phase opposition and the reflector transmits a reflected wave polarized at 90.degree. to the incident wave, which will not propagate in the transmitter-receiver waveguide. It is therefore impossible to use Luneberg reflectors designed for rectilinearly polarized waves to reflect circularly polarized waves.
It has been proposed (see French Pat. No. 1,202,058 filed Sept. 9, 1958) to produce a reflector by applying a series of wires mounted on combs to a viscous material maintained in a mould and subsequently hardened and released from the mould. Such a structure is unsuitable for the reflection of circularly polarized waves because it involves an attenuation of 6 dB, only half the energy being reflected.
There have also been proposed reflectors for circularly polarized waves, notably in French Pat. No. 1,192,598, which consists of plane conductive panels formed with parallel grooves or corugation constituting a reflecting trihedron whose aperture angle is very small.
U.K. Pat. No. 984,144 assigned to TELEFUNKEN discloses a Luneberg reflector for circularly polarized waves comprising a parallel wire grid located preferably at n.lambda./8 or n.lambda./4 in front of the reflection means wherein the gap between the grid wires is smaller than .lambda./2. This patent gives no practical way of producing said reflector and more particularly of laying the grid wires on the inner dielectric sphere.
U.S. Pat. No. 2,989,746 assigned to MARCONI WIRELESS TELEGRAPH COMPANY discloses a scanning antenna system for linearly polarized waves which uses a partially reflecting coating on a complex hollow surface of revolution made of ringlike sections. According to FIG. 3, the coating may consist in a woven cloth made of glass and metal wires at 45.degree. to the length of the step when flat.
It is an object of the invention to provide means to produce wide aperture Luneberg reflectors the aperture of which may be readily adjusted and larger than 120.degree..
It is another object of the invention to provide Luneberg reflectors for circularly polarized waves the equivalent surface of which is equal to that of a reflector of the same dimension for linearly polarized waves. It is another object of the invention to provide a reflector for circularly polarized waves the weight of which is almost equal to that of a reflector for linearly polarized waves.